Recent times have seen an acceleration in efforts by suppliers of consumer electronics to greatly expand the amount and quality of information provided to users. The expanded use of multimedia information in communications and entertainment systems along with user demands for higher quality and faster presentations of the information has driven the communications and entertainment industries to seek systems for communicating and presenting information with higher densities of useful information. These demands have stimulated the development and expansion of digital techniques to code and format signals to carry the information.
With traditional television broadcast systems and other systems used for home entertainment, analog signals fill available bandwidths with single program real time signals in a straight forward format that includes much redundant information as well as much humanly imperceivable information. In contrast, digital transmission systems possess the ability to combine and identify multiple programs and to selectively filter out redundant or otherwise useless information to provide capabilities for the transmission of programs having higher quality or having higher useful information carrying ability or density. As a result of the high technological demand for such capabilities, advances toward the specification and development of digital communications formats and systems have accelerated.
In furtherance of these advances, the industry sponsored Motion Pictures Expert Group (MPEG) chartered by the International Organization for Standardization (ISO) has specified a format for digital video and two channel stereo audio signals that has come to be known as MPEG-1, and, more formally, as ISO-11172. MPEG-1 specifies formats for representing data inputs to digital decoders, or the syntax for data bitstreams that will carry programs in digital formats that decoders can reliably decode. In practice, the MPEG-1 standards have been used for recorded programs that are usually read by software systems. The program signals include digital data of various programs or program components with their digitized data streams multiplexed together by parsing them in the time domain into the program bitstreams. The programs include audio and video frames of data and other information.
An enhanced standard, known colloquially as MPEG-2 and more formally as ISO-13818, has more recently been agreed upon by the ISO MPEG. This enhanced standard has grown out of needs for specifying data formats for broadcast and other higher noise applications, such as high definition television (HDTV), where the programs are more likely to be transmitted than recorded and more likely to be decoded by hardware than by software. The MPEG standards define structure for multiplexing and synchronizing coded digital and audio data, for decoding, for example, by digital television receivers and for random access play of recorded programs. The defined structure provides syntax for the parsing and synchronizing of the multiplexed stream in such applications and for identifying, decoding and timing the information in the bitstreams.
The MPEG video standard specifies a bitstream syntax designed to improve information density and coding efficiency by methods that remove spatial and temporal redundancies. For example, the transformation blocks of 8.times.8 luminance pels (pixels) and corresponding chrominance data using Discrete Cosine Transform (DCT) coding is used to remove spatial redundancies, while motion compensated prediction is used to remove temporal redundancies. For video, MPEG contemplates Intra (I) frames, Predictive (P) frames and Bidirectionally Predictive (B) frames. The I-frames are independently coded and are the least efficiently coded of the three frame types. P-frames are coded more efficiently than are I-frames and are coded relative to the previously coded I- or P frame. B-frames are coded the most efficiently of the three frame types and are coded relative to both the previous and the next I- or P-frames. Headers in the bitstream provide information to be used by decoders to properly decode the time and sequence of the frames for the presentation of a moving picture. The video bitstreams in MPEG systems include a Video Sequence Header containing picture size and aspect ratio data, bit rate limits and other global parameters.
Video images to be viewed by a user are normally produced in a known manner by a scanning process across a video display. The choice of a particular scanning process to be used is generally a design trade off among contradictory requirements of bandwidth, flicker, and resolution. For normal television viewing, generally, an interlaced scanning process uses frames that are composed of two fields sampled at different times. Lines of the two fields are interleaved such that two consecutive lines of a frame, that is, a full display, belong to alternate fields. An interlaced scanning process represents a vertical temporal trade off in spatial and temporal resolution. Thus, slow moving objects are perceived with higher vertical detail, while fast moving objects are perceived with a higher temporal rate, although at half the vertical resolution.
The presentation of MPEG video involves the display of video frames at a rate of, for example, twenty-five or thirty frames per second (depending on the national standard used, PAL or NTSC, for example). Thirty frames per second corresponds to presentation time intervals of approximately 32 milliseconds. Thus, MPEG-2 video decoders must decode signals with interleaved video in what has been called, and referred to above as, the CCIR-601 (and which has also been called the ITU-R) color video format, where each pixel is coded as a luminance 8 bit value sampled at a 13.5 MHZ rate along with a red chrominance value and a blue chrominance value, 8 bits each and sampled at a 6.75 MHZ rate. In this format, the video frames are 720 pels per line, and either 480 lines per frame at 30 frames per second or 576 lines per frame at 25 frames per second.
In contrast to normal television display, computer video terminals often use non-interlaced, that is, progressive or sequential, displays with refresh rates of higher than 60 frames per second, for example, 72 frames per second. Generally, with computer displays, the viewer is sitting closer to the display and the material being displayed is often generally static. Thus, if an interlaced display is used as a computer display, one often experiences a large area of flicker, interline flicker, line crawling and other distractions. Generally, video signal processors are designed to specifically drive either a non-interlaced or an interlaced display monitor.
It is also known, pursuant to the MPEG-2 standard, that different video formats may be utilized in order to reduce the amount of data required. MPEG-2 video coding is optimized for the CCIR-601 4:2:2 interlaced format and, therefore, the 4:2:2 interlaced format is normally used in decoding video signals. In a MPEG-2 4:2:0 video format, the number of samples of each chrominance component, Cr or Cb, is one-half the number of samples of luminance, both horizontally and vertically. In contrast, with the MPEG-2 4:2:2 video format, in each frame of video, the number of samples per line of each chrominance component, Cr or Cb is one-half of the number of samples per line of luminance. However, the chrominance resolution is full vertically, that is, it is the same of that of the luminance resolution vertically. In the normal course of video signal processing, the 4:2:0 format is used, and that format is interpolated to a 4:2:2 format for the video display monitor.
In addition to the above variations, a video signal processor must be able to process video that has been derived from a wide range of sources. For example, the program material may be derived from 16 mm, 35 mm, or 70 mm film, cinemascope film, or wide screen film. Each of those film sources has a different display size, which is often calibrated in terms of its image aspect ratio, that is, the ratio of picture width to height. For example, the aspect ratio of 16 mm film, wide screen film, 70 mm film, and cinemascope film are 1.33, 1.85, 2.10, 2.35, respectively. The aspect ratio of NTSC, PAL, and SECAM TV is 1.33, whereas the aspect ratio for HDTV is 1.78. Given those variations in aspect ratio in combination with different sizes of video displays, it is often required to adjust the horizontal width or vertical height of the displayed image. Thus, the video signal processor must be capable of driving display monitors such that images having different aspect ratios may be displayed.
Known devices for performing a change in aspect ratio generally require a separate specialized processor, additional memory, as well as significant manipulation of the data. Such processors add cost, require too much overhead and introduce too much delay into the video processing function. Often, letterboxing requires that image lines be stored in the DRAM for a period of time while blank lines are being output. Such an operation wastes or does not most efficiently utilize the available memory. Other schemes create the letterbox image and write it to memory, and that stored letterbox video data is post filtered and output. Such a scheme doubles the number of memory reads after the video data has been decoded, thereby consuming valuable processing time. Thus, there is a need for device that more efficiently and economically automatically provides a desired aspect ratio.